Abstract: A light transmitter transmits a beam of light along a measurement path to a light receiver which receives the light beam either directly or via a retro-reflector. The light receiver forms electrical signals representative of the transmission or absorption along the measurement path. In an arrangement with separate light transmitter and receiver units these units are arranged in respective housings at each end of the measurement path. In an alternative arrangement in which the light transmitter and receiver are in a single unit a retro-reflector is positioned at the other end of the measurement path. In this alternative arrangement the light transmitter/receiver unit is embodied in one housing and the retro-reflector in a second housing. In either arrangement both housings are sealingly closed by at least one and preferably two windows which can be moved cyclically into and out of the light beam.